Re: [PATCH] mm/damon: Validate if the pmd entry is present before accessing

[Baolin Wang]

On 8/18/2022 11:39 AM, Muchun Song wrote:
> > On Aug 18, 2022, at 10:57, Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> >
> >
> >
> > 在 8/18/2022 10:41 AM, Muchun Song 写道:
> > > > On Aug 17, 2022, at 14:21, Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > The pmd_huge() is used to validate if the pmd entry is mapped by a huge
> > > > page, also including the case of non-present (migration or hwpoisoned)
> > > > pmd entry on arm64 or x86 architectures. Thus we should validate if it
> > > > is present before making the pmd entry old or getting young state,
> > > > otherwise we can not get the correct corresponding page.
> > > >
> > > > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > > mm/damon/vaddr.c | 10 ++++++++++
> > > > 1 file changed, 10 insertions(+)
> > > >
> > > > diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> > > > index 3c7b9d6..1d16c6c 100644
> > > > --- a/mm/damon/vaddr.c
> > > > +++ b/mm/damon/vaddr.c
> > > > @@ -304,6 +304,11 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,
> > > >
> > > > 	if (pmd_huge(*pmd)) {
> > > > 		ptl = pmd_lock(walk->mm, pmd);
> > > > +		if (!pmd_present(*pmd)) {
> > > Unluckily, we should use pte_present here. See commit c9d398fa23788. We can use
> > > huge_ptep_get() to get a hugetlb pte, so it’s better to put the check after
> > > pmd_huge.
> >
> > IMO this is not the case for hugetlb, and the hugetlb case will be handled by damon_mkold_hugetlb_entry(), which already used pte_present() for hugetlb case.
>
> Well, I thought it is hugetlb related since I saw the usage of pmd_huge. If it is THP case, why
> not use pmd_trans_huge?

IIUC, it can not guarantee the pmd is present if pmd_trans_huge() 
returns true on all architectures, at least on X86, we still need 
pmd_present() validation. So changing to pmd_trans_huge() does not make 
code simpler from my side, and I prefer to keep this patch.

Maybe we can send another cleanup patch to replace pmd_huge() with 
pmd_trans_huge() for THP case to make code more readable? How do you 
think? Thanks.

> > > Cc Mike to make sure I am not missing something.
> > > Muchun,
> > > Thanks.
> > > > +			spin_unlock(ptl);
> > > > +			return 0;
> > > > +		}
> > > > +
> > > > 		if (pmd_huge(*pmd)) {
> > > > 			damon_pmdp_mkold(pmd, walk->mm, addr);
> > > > 			spin_unlock(ptl);
> > > > @@ -431,6 +436,11 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
> > > > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > > > 	if (pmd_huge(*pmd)) {
> > > > 		ptl = pmd_lock(walk->mm, pmd);
> > > > +		if (!pmd_present(*pmd)) {
> > > > +			spin_unlock(ptl);
> > > > +			return 0;
> > > > +		}
> > > > +
> > > > 		if (!pmd_huge(*pmd)) {
> > > > 			spin_unlock(ptl);
> > > > 			goto regular_page;
> > > > --
> > > > 1.8.3.1